Q: The RH range in my home and workshop is roughly from 20 percent (wintertime) to 80 percent (summertime). This gives a range of 4.5 to 16 percent EMC, winter to summer. The Wood Handbook gives the expansion of cherry from bone dry to 30 percent MC of 7.1 percent. In proportion, then, my cherry should change 2.7 percent in size on the average. However, I have never found an expansion higher than 1.3 percent in unfinished wood or oil-finished furniture in my shop and home. I also monitor the top gaps of twenty drawer fronts on furniture in my house (oil finished). They show that the fronts expand by between 0.6 and 1.0 percent. Other fine woodworkers that I know generally work on 1/8 inch per foot, or less, for expansion gaps, i.e. 1 percent or less. I would greatly appreciate any comment you can make on the anomaly between published figures and my measurements.
A: There are at least seven reasons why you notice less shrinkage than predicted from the published values .
First , the values in the Wood Handbook (WH) are for perfectly tangential grain. A flatsawn piece is never perfectly flatsawn, but always will have some quarter (or different than perfectly parallel) grain. Quartersawn grain shrinks about 1/2 of perfect flatsawn grain. Hence, the actual size change will be less than predicted.
Second , the values in the WH are based on very small (thin) pieces of wood. A larger piece of wood (thicker) will have the surface fibers trying to shrink or swell but the core (which does not yet "appreciate" that the EMC has changed on the outside of the piece), will not be shrinking (or swelling). Because the outer fibers cannot move the full amount desired, the shrinkage or swelling is less for a thicker piece than predicted from the thin pieces.
Third , the WH values are for shrinkage only...in fact, shrinkage from a high MC to a low MC the first time that the piece of wood is dried. Therefore, it is incorrect to talk about "expansion" values, although they are often used for predicting expansion as they are close.
Fourth , the values ignore the hysteresis effect, which means that when you are dry and go to a wetter condition, there will be a lag of several percent before swelling starts. Likewise, once you get swelling occurring, there will be a lag when you start drying again.
Fifth , the EMC is not always the MC of wood; there is a hysteresis effect with moisture. So, when the EMC changes, the wood will lag in MC. That is, if the wood is exposed to 65 percent RH (12 percent EMC), but it is coming from a dry condition, the wood will perhaps reach 10.5 percent to 11.0 percent MC, but will never reach 12 percent MC. Then when exposed to 30 percent RH (6 percent EMC), the wood will dry to about 7 percent MC, but not reach 6 percent MC. This is the hysteresis effect for MC, which is added to the hysteresis shrinkage effect.
Sixth , there is a time effect. When the EMC increases, the wood takes time to fully adjust to the new MC. When your shop goes to 20 percent RH (average, for many days and nights), the wood will take time to reach this low MC. Likewise for increases. So, when you say 20 to 80 percent RH, you need to hold these extremes for a long time (weeks) to allow time for the wood to adjust.
Special note : The end grain will change very quickly when the EMC and MC are not equal. Hence, sometimes when the EMC changes and the wood's MC changes in response, the ends will dry out really fast and shrink while the rest of the piece has not started shrinking. This results in stress that can lead to an end check or open glue joint.
A final comment: I doubt that you have 80 percent average RH in your shop. This would result in mold growth in the colder corners, a damp feeling, etc. We likely would have that high average RH along the coast outside, but not in the interior. Likewise, getting 20 percent average RH in your shop would be very rare as you would need to have lots of heat in the wintertime and it would have to be on 24/7. You would notice lots of dust and chapped lips, static electricity and so on.
Have something to say? Share your thoughts with us in the comments below.